This application relates to conversion from an electrical signal to an optical signal and light modulation in response to an electrical signal.
An electrical signal may carry certain information in either digital or analog form. The information can be imbedded in a property of the electrical signal such as the phase or the amplitude. The information in the electrical signal may be created in various ways, e.g., by artificially modulating the electrical carrier, or by exposing the electrical carrier to a medium which interacts with the carrier. In some applications, such information may be transmitted, processed, stored, received, or detected in the electrical domain. For example, electrical cables can be used to transmit information in electrical form; many electronic circuits or processors (e.g., microprocessors) can process information in electrical form; satellites, radars, and wireless telephones or other electronic devices can transmit or receive information in electromagnetic waves traveling in free space without relying on conductive transmission media.
An optical wave may also be used as a carrier to carry information in either digital or analog form. Similar to an electrical signal, an optical carrier wave may be artificially modulated to carry desired information, or may be brought into interaction with an optical medium to acquire certain information indicating a characteristic of that medium. Examples for the artificial optical modulation include optical modulation by using an optical modulator such as an electro-optic material whose refractive index changes with an applied electric field, or modulation of a driving current in a semiconductor gain material (e.g., a multiple-quantum-well gain medium) which amplifies or generates an optical wave. Examples for interaction between an optical wave and an optical medium include measurements of the optical scattering, reflection, or transmission of optical media. Also similar to electrical signals, optical signals may be transmitted in free space or in optical transmission media such as optical waveguides (e.g., optical fibers or planar waveguides formed on semiconductor, glass or other substrates). In addition, optical signals may be processed or manipulated optically by using optical devices and stored in optical storage media.
Signal transmission, processing, or storage in optical domain may have advantages over the electrical counterpart in certain aspects. For example, optical signals are generally immune to electromagnetic interference which often limits the performance of electronic devices. Also, an optical carrier, having a carrier frequency much higher than that of an electrical signal, can carry more information than an electrical carrier due to the wider bandwidth associated with the higher optical carrier frequency. As another example, optical signals can be used allow for easy parallel transmission and processing to further increase the information carrying capacity as well demonstrated by the optical wavelength-division multiplexing (WDM) techniques.
Many applications may be designed to have electrical-optical “hybrid” configurations where both optical and electrical signals are used to explore their respective performance advantages, conveniences, or practical features. Some existing communication systems, for example, transmit signals in the optical form through fiber networks but process the information at a destination in electronic form (e.g., by one or more digital electronic processors). In such and other applications, the electrical-to-optical conversion is needed.